1. Field of the Invention
The present invention relates to a keyboard and particularly to an improved keyboard with a reduced thickness.
2. Brief Discussion of the Related Art
The conventional keyboards now used on computers for data entry have a key module consisting of keys that are movable vertically or through bridge mechanisms. Reference of the vertical moving structure can be found in R.O.C. patent publication No. 582597, entitled “Improved keyboard structure” which has a key module consisting of a first lid, a second lid, an elastic touch member and a conductive element stacking together. The vertical moving keyboard has a shortcoming, namely it has a significant thickness, thus is not suitable for slim and delicate products such as notebook computers. The thin keyboards mostly adopt the bridge mechanism. The key adopted the bridge mechanism has two movable cross brackets that can be moved vertically under compression for an upward displacement and a downward displacement. Compared with the vertical moving key, the thickness of the bridge key is smaller. Reference can be found in R.O.C. patent No. 1220214, entitled “Keyboard having keys movable in stages” which has FIGS. 1, 2 and 3 showing the basic structure of a key module. It includes a plurality of keycaps, a plurality of support structures (bridge elements), a base board, a circuit film (circuit board) and an elastic film consisting of a plurality of elastic elements stacking together. The base board includes a conductive plate (coupling with a bridge board of the bridge elements) and a base board made of metal. The circuit film is sandwiched between the conductive plate and the metal base board. As the moving displacement of the bridge mechanism is smaller than the vertical moving type, the total thickness of the keyboard is smaller. While the conventional bridge mechanism can reduce the thickness of the keyboard, the thickness of the bridge board takes a significant portion of the thickness of the key module. To remedy this problem, another metal base board was developed that has a plurality of pivotal troughs formed by stamping to hold the bridges and replace the bridge board. R.O.C. patent publication No. 471689, entitled “Improved keyboard key positioning structure” discloses a technique that does not have the bridge board. It has a metal base board stamped to form a plurality of pivotal troughs, and with a circuit film bonding to the metal base board to make the key module slimmer and lighter. But with the metal base board replacing the bridge board also has disadvantages. The conventional bridge board usually is made from plastic injection, thus the cost and technique are lower, and production yield is higher. The metal base board formed by stamping costs higher cost and requires a more sophisticated technique. Production yield is lower. Moreover, in the event that an excessive error happens to any of the pivotal troughs, the entire base board becomes useless and has to be discarded. Hence the average cost of each key module is higher. Moreover, the plastic bridge is easily worn out caused by friction against the metal base board. The life span of the bridge element is lower. At present the keyboard equipped with the bridge mechanism still adopts the bridge board or the base board to replace the bridge board. Either type has its benefits and drawbacks. Nowadays slim and light have become norm of the key module. The main factor that affects the thickness of the keyboard is the thickness of the upper lid and the lower lid. The total thickness of the upper lid and lower lid almost equals to the thickness of the keyboard. Hence the upper lid and lower lid that form the border of the keyboard become a hindrance to further reduce the keyboard thickness.